In general, an ethylene-vinyl acetate copolymer (abbreviated as EVA) blended with organic peroxide has so far been used as a material for forming a sheet for sealing a solar cell because it is flexible and has a high transparency and has excellent long-term durability when blended with additives such as an appropriate weather resistant stabilizing agent, an adhesion promoter and the like.
However, EVA has a low melting point and involves problems such as poor heat resistance which brings about thermal deformation at an environmental temperature at which a solar cell module is used, and therefore a cross-linked structure is formed in EVA by blending it with organic peroxide, whereby a sheet for sealing a solar cell having heat resistance is provided.
Publicly known sheet molding methods by which polyolefins can be molded are used for forming a sheet for sealing a solar cell, but has the problem that if it is blended with organic peroxide as described above, it is forced to be molded at low temperature in order to prevent the organic peroxide from being decomposed and a high speed productivity is inhibited.
Further, in producing a solar cell having a constitution of a surface protective layer (glass, plastics), a sheet for sealing a solar cell, a power module, a sheet for sealing a solar cell, and a surface protective layer (glass, plastics), two steps of a temporary adhesion step by vacuum heat lamination and a cross-linking step by organic peroxide using a high temperature oven are usually employed. Several tens of minutes are required for the above cross-linking step by organic peroxide, and therefore shortening of time in the cross-linking step or elimination of the cross-linking step itself is strongly required.
Further, there exists the concern that when a sheet for sealing a solar cell formed from EVA is used over a long period of time, an adverse effect is exerted on a power module due to the decomposition gas (acetic acid gas) of EVA or a vinyl acetate group present in EVA itself to reduce power generation efficiency.
Accordingly, a sheet for sealing a solar cell which is prepared by using an ethylene•α-olefin copolymer in place of EVA is proposed in order to avoid the problems of the EVA-made sheet for sealing a solar cell described above (Japanese Patent Application Laid-Open No. 2000-91611 (patent document 1)). It is expected that an adverse effect exerted on a power module is reduced by using the ethylene•α-olefin copolymer, but the ethylene•α-olefin copolymer is inferior in a balance between heat resistance and flexibility and does not exhibit good heat resistance if it is not cross-linked, and therefore it has been difficult to produce a sheet for sealing a solar cell by eliminating a cross-linking step. Patent document 1: Japanese Patent Application Laid-Open No. 2000-91611